Where—
an instrument is made varying any of the dispositions of the property comprised in the estate of a deceased person immediately before his death,
the instrument contains a statement under subsection (2) of section 142 above, and
the variation results in additional tax being payable,
the relevant persons (within the meaning of that subsection) shall, within six months after the day on which the instrument is made, deliver a copy of it to the Board and notify them of the amount of the additional tax.
To the extent that any of the relevant persons comply with the requirements of this section, the others are discharged from the duty to comply with them.